1. Field of the Invention
The invention relates to a lock open mechanism for a downhole safety valve, and particularly to a safety valve having an axially shiftable valve head for closing the bore of the well conduit.
2. Summary of the Prior Art
Lock open devices for safety valves have been known in the prior art for practically as long as downhole safety valves. The need for a lock open device is well known. In the event of any failure of the downhole safety valve, a lock open device will permit the bore of the well conduit in which the safety valve is mounted to be open, permitting the passage of well tools downwardly through the defective safety valve. In the event that there are an insufficient number of operative safety valves in the well conduit, the function of the defective safety valve can be performed by an in-tubing safety valve which is lowered through the locked open safety valve to sealingly engage the bore of the defective safety valve and provide a substitute functioning valve at that position in the well conduit. A typical lock open mechanism for a safety valve, and a replacement in-tubing safety valve, are shown in U.S. Pat. No. 3,696,868.
The great majority of safety valves employ either a flapper or a rotatable ball as the valve head which is shiftable between a closed to an open position by a lock open mechanism. Typically, such safety valves are normally operated by an actuating sleeve and the lock open mechanism functions to depress the actuating sleeve to its valve opening position and latch the actuating sleeve in such position.
Recently, safety valves have been developed employing an electric motor or solenoid for axially shifting a valve head between a closed and an open position relative to the bore of the well conduit. See, for example, U.S. Pat. No. 4,796,708. To apply a lock open mechanism to a safety valve of this construction requires an operable engagement between the shiftable valve head and the lock open mechanism. It is highly desirable that the lock open mechanism be insertable by a wireline operation, hence the only downward force available from a wireline suspended lock open mechanism is that produced by jars incorporated in the wireline. This necessarily means that a sustained downward force cannot be applied to the lock open mechanism to effect the shifting of the valve head of the safety valve from its closed to its open position in a single movement of the jar mechanism.
There is, therefore, a definite need for a wireline operated lock open mechanism which will effect the axial shifting of the valve head of a downhole safety valve in successive increments in response to downwardly produced jarring forces generated by a set of jars in the wireline.